1) Field of the Invention
The present invention relates to an inner-focus type telephoto lens that is most suitable for a digital camera and to a telephoto lens apparatus that is most suitable for a lens-exchange type digital camera.
2) Description of Related Art
Conventionally, a number of inner-focus type telephoto lenses have been proposed as exchangeable lenses used for single-lens reflex cameras for silver halide photography. This type of telephoto lens, which performs focusing from the infinity to the proximity by moving inner lenses alone while keeping the total length of the optical system constant, has been proposed by Japanese Patent Application Preliminary Publication (KOKAI) No. Sho 55-147606, Japanese Patent Application Preliminary Publication (KOKAI) No. 2000-89103, Japanese Patent Application Preliminary Publication (KOKAI) No. Sho61-215513, etc. in various configurations.
A telephoto lens as described in JP KOKAI No. Sho 55-147606 or JP KOKAI No.2000-89103 includes, in order from the object side, a first lens unit having a positive refracting power, a second lens unit having a negative refracting power, and a third lens unit having a positive refracting power. This telephoto lens is configured to perform focusing by moving the second lens unit or the third lens unit. In addition, the first lens unit having a positive refracting power is composed of two positive lenses and one negative lens.
Also, a telephoto lens as described in JP KOKAI No. Sho 61-215513 includes, in order from the object side, a first lens unit having a positive refracting power, a second lens unit having a negative refracting power, a third lens unit having a positive refracting power, and a fourth lens unit having a negative refracting power. This telephoto lens is configured to perform focusing via the second lens unit having a negative refracting power and the fourth lens unit having a negative refracting power, and is improved to keep good performance in a closer proximity.
On the other hand, in accordance with recent development of digital cameras, lens-exchangeable digital cameras have been commercialized. Such a digital camera often employs, as its exchange lens, an optical system for silver halide photography as it is.
However, digital cameras having image pickup elements with much greater number of pixels have come on the scene in accordance with engineering improvement of image pickup elements. In order to match such a high-definition image pickup element, optical systems also are required to have much higher resolution. Therefore, if an optical system that has been used for a conventional single-lens reflex camera for silver halide photography is used as a telephoto lens for a digital camera having such a high-definition image pickup element, a sufficiently high resolving power cannot be attained.
In addition, with the arrival of new users of digital cameras, requirement for performance has been much increased. To be specific, reduction of distortion, reduction of chromatic aberration, and, further, keeping of good performance in a closer proximity are required.
In this regard also, it is unsatisfactory to use an optical system that have been used for a conventional single-lens reflex camera as a telephoto lens.